Electrical cables which are used in oil wells must be able to survive and perform satisfactorily under extremely adverse conditions of heat and mechanical stress. Ambient temperatures in wells are often high and the I.sup.2 R losses in the cable itself add to the ambient heat. The service life of a cable is known to be inversely related to the temperature at which it operates. Thus, it is important to be able to remove heat from the cable while it is in its operating environment.
Cables are subjected to mechanical stresses in several ways. It is common practice to attach cables to oil pump pipes to be lowered into a well using bands which can, and do, crush the cables, seriously degrading the effectiveness of the cable insulation and strength. The cables are also subjected to axial tension and lateral impact during use.
It is therefore conventional to provide such cables with external metal armor and to enclose the individual conductors within layers of materials chosen to enhance the insulation and strength characteristics of the cable, but such measures are sometimes not adequate to provide the necessary protection.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. Typically, the insulation surrounding the conductors in a cable contains micropores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the wall, there is not sufficient time for the intrapore pressure to bleed off. As a result, the insulation tends to expand like a balloon and can rupture, rendering the cable useless thereafter.
In U.S. Pat. No. 4,409,431 in which the assignee is the same as the assignee of the instant invention, there is described a cable structure which is particularly suitable for use in such extremely adverse environments. The structure protects the cable against compressive forces and provides for the dissipation of heat from the cable which is an important feature in high temperature operating environments, for reasons discussed therein.
As described in said copending application, the cable protective structure includes one or more elongated support members which conform to, and extend parallel and adjacent an insulated conductor comprising the cable. The members are rigid in cross-section to resist compressive forces which would otherwise be borne by the cable conductors. For applications requiring the cable to undergo long-radius bends in service, the elongated support may be formed with a row of spaced-apart slots which extend perpendicularly from the one edge of the support member into its body to reduce the cross-sectional rigidity of the member in the slotted areas so as to provide flexibility in the support to large-radius bending about its longitudinal axis.
For certain service applications, it may be preferred that the electrical insulating jacket on a cable conductor not be in direct contact with the slot openings. This is because the slot openings in the support member may allow highly corrosive materials to gain access to the jacket composition by flowing inwardly through the slots. In addition, the corners formed by the slots may cut into or abrade the underlying cable jacket upon repeated bending of the cable.